TL;DR: Specifically for Zoom, take a look at Zoom Meeting Connector
First off, to get it out of the way, encrypted is not the same as secure, and secure can be vague depending context.
As schroeder♦ have commented, you need to be clear on what you are actually trying to achieve, what threats you are defending against. Only then you may determine if a solution really solve your problem. It might turn out E2E encryption isn't what you actually need, or want. And like Steffen Ullrich said, you can't just magically add that without significant changes to both its application and infrastructure.
Fortunately, in the case of Zoom, there is a relatively easy way out (depending on you actual needs). Zoom allows you to run your own server for streaming audio and video, while still using Zoom server for other management tasks.
Zoom offers a public or hybrid cloud service. In the hybrid cloud service, you deploy meeting communication servers known as the Zoom Meeting Connector within your company's internal network. In doing so, user and meeting metadata are managed in the public cloud while the meetings are hosted in your private cloud. All meeting traffic including video, voice and data sharing goes through the on-premise Zoom Meeting Connector.
This way, the conference data stays in a server you control. Even if you makes call from outside internal network, the traffic is still encrypted in transit and only decrypted on (your) meeting server. Deployment should not be difficult for a corporate IT team, though might be challenging for laymen. If you want the privacy of E2EE, this is about as close as you can get without actually changing the software or rolling you own service.